An Introduction to a Biology 



It is sometimes easy to determine the extent of these 

 units, as in the case of discontinuous characters such as 

 purpleness and whiteness of flower in Pisiim : ^ it is often 

 difficult, as in the case of characters varying continuously 

 such as the weight of beans. ^ 



Careful consideration of the table in my paper ^ 

 on the supposed antagonism of Mendelian to biometric 

 theories shows that the accuracy with which Mendelian 

 or Galtonian Law describes the facts of heredity depends 

 on the composition of the unit tested. There is nothing 

 a priori illogical in treating the sharply defined category 

 of dark-eye-and-coloured-coat, in mice, as a unit. Suppose 

 this is done : Galton's Law fits the facts beautifully, while 

 Mendel's is triumphantly refuted (by showing that the 

 amount of albino ancestry of a hybrid affects the percentage 

 of albinos produced by such hybrids mated inter se) ; to 

 which the Mendelian would make the follomng answer : 

 " Are you sure that your unit ' dark-eye-and-coloured- 

 coat' is incapable of resolution into still simpler units ? Are 

 you sure that you are not regarding as a simple thing that 

 which is really compound, just as the ' fixed alkalies ' were 

 regarded as elements until Davy * showed them to be com- 

 pounds ? ^ I can prove that you are ; for by testing the 

 gametic constitution of the dark-eyed and coloured-coated 

 forms I can show that they are sharply distinguished into 

 heterozygous and homozygous forms. Now I claim to have 

 discovered what should really be treated as a unit, namely, 



^ Fruwirth, :06, p. 141. 2 Johannsen, :03. 



^ Darbishire, :05a, vide supra, p. 148. * Davy, '08. 



^ This illustration may at first sight appear not to be strictly parallel. 

 A chemist reading it would think that what was going to be shown was 

 that the unit " dark-eye-and-coloured-coat " was resolvable into " dark- 

 eye " and " coloured-coat," whereas, of course, what is really going to be 

 shown is that the units " dark-eye " and "coloured-coat " are of two quite 

 distinct kinds. The true parallel to my case is the idea of the element 

 before Davy's time. Amongst the things that were classed as elements, 

 some really were undecomposable {cf. the homozygous forms), while others 

 — the alkaUes — were decorapoaable {cf. the heterozygous). 



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